Paul Rudd was clueless about co-star Selena Gomez’s superstardom

CHICAGO, IL – JUNE 25: Selena Gomez performs during The Revival Tour at United Center on June 25, 2016 in Chicago, Illinois. (Photo by Daniel Boczarski/Getty Images)

PAUL RUDD had no idea how big a star SELENA GOMEZ was until he filmed new movie THE FUNDAMENTALS OF CARING with her.

The “Ant-Man” star knew his castmate was popular, but had no idea she was a really big deal until he witnessed her fame at first hand.

“I’m so out of touch,” Paul admits to Variety. “It wasn’t until we were filming and I saw thousands of people turning out to try and see her that I realized what a phenomenon she is. To see her deal with that kind of fame, I came away even more impressed with her. She’s extraordinary.”

Director Rob Burnett also praised the 23-year-old’s talents, saying Selena is “only at the beginning of her talents.”

In the film, the “Ant-Man” star plays an out-of-work novelist who is charged with looking after a foul-mouthed, wheelchair-bound teenager on a road trip. Selena plays a young runaway who joins their adventure.

And while Paul was left in awe by Selena’s acting skills, he also enjoyed their time together off-screen – she previously told Vulture that during their nights off, they would bond by hunting for ghosts using an app that allows wannabe spook hunters to track down ghouls.

“The most memorable experiences I had with the gang were just being in random motels, hotels, abandoned places…” Selena said. Paul added, “Those are good bonding times for all of us sitting in this hotel room.”

“And then we got the ghost app, remember?” Selena giggled. “We were hunting ghosts, and he humored me so I could believe it was real.”

But superstitious Selena is convinced they really did stumble upon something from the other side, adding, “I think her name was Cathy or something?” Paul admitted he was instantly hooked on the ghost finder app: “You get it on your phone and if you’re near a ghost, it would tell you.”

“At 3am anything is entertaining,” Selena laughed.

“The Fundamentals of Caring” received positive reviews when it screened at the Sundance Film Festival earlier this year and it is now available to stream on Netflix.